Membrane-Based Chemical Separation (MBCS) systems have experienced rapid growth in U.S. industries (e.g., Monsanto, DuPont, Dow, and Union Carbide) over the past two decades and are used to separate and purify many high volume chemicals. Broadly stated, a membrane is a barrier material that allows for selective transport. High-profile chemical separations achieved with state of the art commercial membrane systems include: (1) size dependent particle removal filtration from fluid streams; (2) water desalination by reverse osmosis; (3) organic/organic separations and solvent dehydration by pervaporation; and (4) gas separations for hydrogen recovery, nitrogen production, and acid gas removal from natural gas streams. New materials with higher rates and selectivities are required by industry to advance MBCS technologies. MBCS systems permit: (a) high throughput and high selectivity to efficiently concentrate individual components from complex mixtures; (b) small numbers of mechanical parts; and (c) low capital and operating costs due to significantly reduced energy requirements. Because of these advantages, MBCS systems occupy an important place in separation technologies and are attractive for commercial process operations.
The process of separating pure components from a mixture of gases is of great industrial importance. Current gas separation technologies have several shortcomings which include high capital costs, poor energy efficiency, and generation of secondary pollution. Two principal gas separation techniques in use today are cryogenic distillation and pressure-swing gas adsorption. Membrane-based separations have more recently been proven to be economically competitive with these methods while possessing several advantages including reduced energy costs and large reductions in secondary pollution. Moreover, improvements in membrane separation systems may lead to significant savings for the energy industry. Advanced membrane systems for enriched oxygen and nitrogen production alone could save 0.4 quad units of energy per year. See, e.g., “Membrane Separation Systems”, U.S. Department of Energy, DE 90-011771 (1990). New membrane systems needed to advance gas separation technology should be: (a) environmentally stable; (b) easy to manufacture and maintain; (c) low in cost; and, most importantly, (d) capable of combining high gas selectivity with high gas flux.
Air separation systems based on gas selective polymer membranes have now been commercialized by several U.S. companies. These are important separations since nitrogen and oxygen are the second and third largest commodity chemicals produced in the United States. See, e.g., J. Huggin, Chem. Eng. News 66, 7 (1988). The first commercial air separation systems used conventional engineering polymers for O2/N2 separations with selectivities in the range of 2-4. Current technology employs membranes with separation factors of 6-7, and an oxygen permeability of 2 Barrers. See, e.g., U.S. Pat. No. 4,840,646 which issued to J. N. Anand et al. on Jun. 20, 1989. A recent report prepared by the U.S. Department of Energy states that if stable membranes are developed with an oxygen/nitrogen separation factor of 20 and an oxygen permeability of 10 Barrers, such systems would completely displace present day cryogenic air separations. A membrane with these qualities would readily allow compressed air to be enriched to >75% oxygen or produce nitrogen of >99.9% purity. Energy costs saved in oxygen and nitrogen production, and those saved from enhanced combustion processes, would equal the energy equivalent to 105 to 106 barrels of oil per day. See, e.g., “Membrane Separation Systems: A Research Needs Assessment” by W. Koros, U.S. Department of Energy, DE 90-011770 (1990). Other important commercial gas separations include the recovery of hydrogen and the separation of nitrogen, helium, hydrogen, and carbon dioxide from natural gas.
The conjugated polymer polyaniline is stable in both its undoped and doped states and has a simple and completely reversible acid/base doping chemistry. Permeability tests on thick (25 to 50 μm) as-cast polyaniline films have shown that gases of different kinetic diameters permeate at different rates leading to high selectivity ratios for important gas pairs such as oxygen/nitrogen, hydrogen/nitrogen, and carbon dioxide/methane. See, e.g., “Conjugated Polymer Films For Gas Separations” by M. R. Anderson et al., Science 252, 1412 (1991). By partially doping polyaniline, a highly selective membrane was formed. Two recent papers have confirmed that partially doped polyaniline has the highest known oxygen/nitrogen selectivity and lies above an “upper bound” permselective behavior for both glassy and rubbery polymers. See, e.g., S. Kuwabata and C. R. Martin, J. Membrane Sci. 91, 1 (1994) and L. Rebattet et al., J. Appl. Polym. Sci. 57, 1595 (1995). The commercial potential for gas-selective polyaniline membranes has been limited, however, since there are no reports where a sufficiently thin skin (<1 μm) has been generated to achieve commercially significant gas transport rates. Since gas flux is inversely proportional to the thickness of a barrier membrane, the thinnest possible nonporous polymer layer is desired. Suitable membranes must also be able to withstand pressures of up to 100 atmospheres or more. Separations are therefore generally carried out using asymmetric membranes in which a thin layer or skin of polymer is grown on a porous structural support (asymmetric composite), or where both the thin separating layer and the porous support are the same polymer (integrally skinned asymmetric membrane or ISAM).
Accordingly, it is an object of the present invention to provide a robust, stable permeable polyaniline membrane having a significant gas transport rate and significant gas transport selectivity.
Another object of the present invention is to provide a robust, stable permeable polyaniline hollow fiber having a significant gas transport rate and significant gas transport selectivity.
Still another object of the invention is to provide a robust, stable permeable polyaniline membrane having a significant gas transport rate and oxygen/nitrogen, hydrogen/nitrogen and carbon dioxide/methane gas selectivities.
Yet another object of the present invention is to provide a robust, permeable polyaniline hollow fiber having a significant gas transport rate and oxygen/nitrogen, hydrogen/nitrogen and carbon dioxide/methane gas selectivities.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.